A difficult sealing challenge is presented when pressurized fluid must travel across the annular interface formed between a pair of coaxial, relatively rotating cylindrical components. An example of such an environment can be found in a typical vehicle automatic transmission. Various hydraulically operated clutches in the system include cylindrical drums that closely surround the outer surface of an input shaft, forming a thin, annular interface. The pressurized hydraulic fluid needed to operate the clutch piston is supplied by an oil pump, and travels down the center of the input shaft until it reaches the point where the clutch drum surrounds the shaft. From there, a cross drilled feed passage takes the fluid through the shaft and to the interface. From there, the pressurized fluid travels through a port in the piston drum, so as to supply the pressure necessary to operate the piston. Obviously, it is important to prevent too much of the pressurized fluid from leaking into the interface if enough pressure is to be left for the piston.
To prevent fluid loss, a pair of circular grooves is cut into the surface of the input shaft bordering the outlet of the pressurized fluid feed passage, each of which holds an annular seal ring. The sides of the seal rings are flat, so as to closely engage the side walls of the grooves, and the free state diameter of the seal ring is close to the diameter of the inner surface of the piston drum, against which it must seal. In addition, the seal ring is often cut at one point, so that the diameter of the ring can expand and contract to accommodate temperature expansion and contraction, or eccentricity between the shaft and drum, thereby maintaining the radial continuity of the seal. The ring may be cut on a diagonal, so as to create two sloped- or wedge-shaped free ends that overlap and slide back and forth over one another, maintaining the circumferential continuity of the seal ring as it expands and contracts. One drawback of this type of seal ring is that it works best only if there is no axial gap between the seal rings and the side walls of the grooves when the pressurized fluid first leaves the feed passage. If contact is not complete, then pressurized fluid can find its way out the gap and into the interface, threatening the axial continuity of the seal. Likewise, in the case where the seal ring has overlapping, slanted ends, part of the seal may tend to get pried away from the side wall of the groove by the underlying sloped end, also threatening the axial continuity of the seal.